This invention relates to an optical sensor useful for detecting, quantifying or differentiating chemical species in gaseous or liquid states, as well as an apparatus for detecting chemical species using said optical sensor.
Various types of optical sensors using planar optical waveguides have been known. For example, Tiefenthaler and Lukosz reported on the use of optical grating couplers as biochemical sensors (J. Opt. Soc. Am., B6 (1989) 209 and Sensors and Actuators, 15 (1988) 273). The sensors make use of the angular resonance condition of the excitation of a certain mode of light in a planar waveguide by a grating coupler. The adsorption of human immunoglobulins on the grating coupler changes the thickness and refractive index of the waveguide and, hence, the coupling angle of the light incident on or emerging from the waveguide. Thus, the measurement system under consideration utilizes the effect that the thickness and refractive index of an optical waveguide change upon adsorption of biochemical molecules on top of the waveguide. Such changes can be detected by measuring the coupling angle in the case where a grating coupler or a prism coupler is used. This detection corresponds to the measurement of effective refractive index of certain modes.
Reuter and Franke reported in Appl. Phys. Lett., 52 (1988) 678 a planar optical waveguide having a birefringent polyimide film for monitoring humidity. Light in TE and TM modes will be launched simultaneously into the planar waveguide by means of a prism coupler. The reported measurement system measures the difference between the effective refractive indices in TE and TM modes. This measurement requires special birefringent films. The output obtained by the measurement is a periodic function of the ambient humidity. If one wants to use the output in a certain application, for example, process control, a complex numerical operation system is necessary.
Also known are various optical sensors that utilize the swelling of thin polymer films due to the absorption or adsorption of gases or liquids. For example, Gauglitz et al. reported on a reflection spectroscopy method for gas or solvent detection via swelling of a polymer film (GIT Fachz. Lab., 7(1990) 889 and Abstracts of 1st European Conference on Optical Chemical Sensors and Biosensors, p. 143 (1992)). The setup for implementing the method uses a white light source and a spectrometer for analyzing spectrally the changes in reflected light.
Nylander et al. reported on a setup using the surface plasmon resonance method for gas detection in Sensors and Actuators, 3 (1982/3) 79. When implementing this method, the resonance condition for surface plasmons is greatly influenced by optical parameters of the polymer film used which, in turn, will change with the surrounding organic vapor.
Butler reported a sensor with a polymer film deposited at an end of a multimode optical fiber (Chemical, Biochemical and Environmental Fiber Sensor II, Proc. SPIE 1368 (1990) 46-54). Upon contacting chemical solvents, the polymer film swells to cause a change in reflectance at the fiber end.
The conventional sensors described above utilize the swelling of thin polymer films and detect it by various methods, including interference enhanced reflection (IER) and surface plasmon resonance for measuring the changes in the thickness or refractive index of the thin polymer film.